<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"> 
<title>CSS基本语法</title> 
<style>
@media screen and (max-width: 500px){
    body{
        background-color: aqua;
    }
}

</style>
</head>
<body>
    <picture>
        <source srcset="img_smallflower.jpg" media="(max-width: 400px)">
        <source srcset="img_flowers.jpg">
        <img src="img_flowers.jpg" alt="Flowers" style="width:auto;">
    </picture>
    <video width="400" controls>
        <source src="mov_bbb.mp4" type="video/mp4">
        <source src="mov_bbb.ogg" type="video/ogg">
        Your browser does not support HTML5 video.
    </video>
</body>
</html>